
LT3724
11
3724fd
OPERATIONS
Soft-Start
The soft-start function controls the slew rate of the power
supply output voltage during start-up. A controlled output
voltagerampminimizesoutputvoltageovershoot,reduces
inrush current from the VIN supply, and facilitates supply
sequencing. A capacitor, CSS, connected between VOUT of
the supply and the CSS pin of the IC, programs the slew
rate. The capacitor provides a current to the CSS pin which
is proportional to the dV/dt of the output voltage. The
soft-start circuit overrides the control loop and adjusts the
inductor current until the output voltage slew rate yields a
2Acurrentthroughthesoft-startcapacitor.Ifthecurrentis
greater than 2A, then the current threshold set by the DC
control voltage, VC, is decreased and the inductor current
is lowered. This in turn lowers the output current and the
output voltage slew rate is decreased. If the current is less
than 2A, then the current threshold set by the DC control
voltage, VC,isincreasedandtheinductorcurrentisraised.
This in turn increases the output current and the output
voltage slew rate is increased. Once the output voltage is
within 5% of its regulation voltage, the soft-start circuit
is disabled and the main control regulates the output. The
soft-start circuit is reactivated when the output voltage
drops below 70% of its regulation voltage.
Slope/Antislope Compensation
The IC incorporates slope compensation to eliminate
potential subharmonic oscillations in the current control
loop. The IC’s slope compensation circuit imposes an
artificial ramp on the sensed current to increase the rising
slope as duty cycle increases.
Unfortunately, this additional ramp typically affects the
sensed current value, thereby reducing the achievable
current limit value by the same amount as the added ramp
represents. As such, the current limit is typically reduced
asthedutycycleincreases.TheLT3724,however,contains
antislope compensation circuitry to eliminate the current
limitreductionassociatedwithslopecompensation.Asthe
slope compensation ramp is added to the sensed current,
a similar ramp is added to the current limit threshold. The
end result is that the current limit is not compromised so
the LT3724 can provide full power regardless of required
duty cycle.
Shutdown
The LT3724 includes a shutdown mode where all the
internal IC functions are disabled and the VIN current is
reduced to less than 10A. The shutdown pin can be used
forundervoltagelockoutwithhysteresis,micropowershut-
downorasageneralpurposeon/offcontroloftheconverter
output. The shutdown function has two thresholds. The
first threshold, a precision 1.23V threshold with 120mV
of hysteresis, disables the converter from switching. The
second threshold, approximately a 0.7V referenced to
SGND,completelydisablesallinternalcircuitryandreduces
the VIN current to less than 10A. See the Application
Information section for more information.
(Refer to Functional Diagram)